1. Field of the Invention
The invention relates to an apparatus for optical recording, reproducing and erasing data into or from a recording medium by radiating a laser beam, and more particularly to such an apparatus having a function of properly radiating a laser beam to a recording medium by compensating for a focal distance of a laser beam in accordance with a thickness of a recording medium.
2. Description of the Related Art
A system for recording data into an optical disc by radiating a laser beam makes it possible to record a large amount of data, and make quick access to data without any mechanical contact, and hence, has been put to practical use as a large capacity memory. An optical disc may be grouped into many types such as a read-only type used only for reproducing data therefrom, such as a compact disc and a laser disc, a write-one type by which a user can additionally record data thereinto, and a rewritable type by which a user can newly record data thereinto and erase data therefrom. The write-once type and rewritable type optical discs are used an external memory for a computer, or as files to store documents and/or images therein.
In a presently used optical disc, a reproduction signal is detected from a laser beam having been modulated at and reflected from an optical disc. For instance, in an optical disc from which data is reproduced only, a reproduction signal is detected by means of concave and convex pits which are formed at a surface of an optical disc and by which reflected light is varied in amount. In an optical disc into which a user can newly record data, data is reproduced by detecting a variation in an amount of a reflected light, caused by phase change or fine pits formed at a surface of an optical disc by radiation of laser thereto.
In a magneto-optic disc which one of rewritable type discs, magnetization of a record film is detected as a variation in a polarizing plane by virtue of magneto-optical effect of the record film. In a phase-change type optical disc which is also one of rewritable type discs, data is reproduced by detecting a variation in an amount of reflected light, caused by a phase-change, similarly to an optical disc into which a user can newly record data.
As an optical disc, a 1.2 mm-thick disc such as a compact disc (CD) and CD-ROM has been used. However, in these days, there has been developed highly densified optical discs such as DVD, DVD-ROM, and rewritable DVD-RAM. In such optical discs, a laser beam is designed to have a shorter wavelength and focused with a lens having a greater numerical aperture (NA) in order to make it possible to record, reproduce and erase data at a density as high as possible, in which case, a 0.6-mm thick disc has been employed for reducing aberration caused by inclination of an optical disc.
A magnetically modulated type magneto-optic disc which is of photo-electromagnetic record type is designed to be made of a substrate having a thickness of 0.8 mm.
Thus, there has been proposed a plurality of optical discs which have a similar shape, but have different thickness.
A user would expect to record, reproduce and erase data into or from a plurality of optical discs with a single disc drive. That is, it is preferable for a user to record, reproduce and erase data into or from optical discs having different thickness by means of a common disc drive.
However, an optical disc is accompanied with a problem as follows. A laser beam is focused through a disc substrate in an optical disc. Hence, when a laser beam is to be passed through a plurality of optical discs, a laser beam cannot be properly focused by means of a single focusing lens, if the optical discs have different thickness.
In order to solve such a problem, there have been suggested many optical heads including means for compensating for a difference in a thickness of disc substrates.
For instance, Japanese Unexamined Patent Publication No. 8-235630 has suggested an optical head including a plate in a disc tray for compensating for a difference in a thickness of disc substrates.
Japanese Unexamined Patent Publication No. 8-138262 has suggested an optical pick up compensating for focusing characteristic thereof by providing a variable aperture system between an objective lens and a laser diode.
Japanese Unexamined Patent Publication No. 7-65409 has suggested an optical data recording and reproducing device compensating for focusing characteristic thereof by providing a convex lens between an objective lens and a laser diode.
Japanese Unexamined Patent Publications Nos. 7-105566 and 6-282866 have suggested an optical pick-up apparatus and an optical disc apparatus including a member or a transparent plate having a variable index of refraction or a variable thickness and located in a convergent optical path between an objective lens and a disc.
Japanese Unexamined Patent Publication No. 4-95224 has suggested an optical disc apparatus having a plurality of focusing optical systems.
Japanese Unexamined Patent Publication No. 6-325405 has suggested an optical data-reproducing apparatus including a first light source for emitting a first laser beam having a first wavelength, a second light source for emitting a second laser beam having a second wavelength longer than the first wavelength, and an optical system for introducing the first and second laser beams to a recording medium, and introducing the first and second laser beams reflected from the recording medium, to a photodetector. When the recording medium is a highly densified one, data is recorded to the recording medium by the second laser beam and data is reproduced from the recording medium by the first laser beam. When the recording medium is a low densified one, data is recorded to and reproduced from the recording medium by the second laser beam.
When optical discs having different thickness are to be used in the above-mentioned apparatuses, a thickness of an optical disc is recognized as follows. If an optical disc is encased in a cartridge, an identification hole formed with a cartridge is detected to thereby recognize a thickness of an optical disc. As an alternative, a bar code which is written on an optical disc and represents data about a thickness of an optical disc is read out to thereby identify a thickness of an optical disc.
However, if an optical disc is not encased in a cartridge, or if a bar code representing data about a thickness of an optical disc is not written on an optical disc, it is impossible to identify a thickness of an optical disc.
As mentioned above, if a thickness of an optical disc is not identified, it is not possible to focus a laser beam onto an optical disc by means of a common focusing lens.